This invention relates to the purification of monochlorobenzene and in particular to the removal of partly chlorinated aliphatic contaminants from monochlorobenzene.
Monochlorobenzene is commonly employed as an intermediate in the production of various chemical compounds and as a solvent in various chemical manufacturing processes, as well as a solvent or carrier for other chemicals in specific applications such as herbicidal applications. Standards for the use of monochlorobenzene as a solvent for herbicidal use require a relatively pure material containing for example less than 50 parts per million chlorinated aliphatic materials.
Monochlorobenzene produced by direct ring clorination of benzene with chlorine commonly contains up to about 0.4 weight percent perchloroethylene. Monochlorobenzene produced by the oxychlorination of benzene with hydrogen chloride gas and oxygen or air, commonly contains minor amount of chlorinated aliphatics, notably perchloroethylene and to a lesser extent, trichloroethylene, typically in amounts of up to about 2% and about 0.1% by weight respectively. Benzene, dichlorobenzene, and higher chlorinated benzens are also commonly produced as by-products in the production of monochlorobenzene. The latter contaminants, that is benzene, dichlorobenzene, and higher chlorinated benzenes are readily separated from monochlorobenzene by distillation. However, the removal of perchloroethylene from monochlorobenzene presents a specific problem. The relatively volatility of perchloroethylene to monochlorobenzene at low concentration was found to be only 1.4, even though there is a difference of 11.3 degrees Celsius in their boiling points. As a result the separation of this component by distillation requires the use of an expensive high efficiency distillation column with a large number of plates and a high reflux ratio. It is known that perchloroethylene can be separated from monochlorobenzene using an azeotropic agent, such as methanol. However, it has been found that a large excess of methanol, for example, in the amount of 200:1 weight ratio of methanol to perchloroethylene is necessary to achieve an effective azeotropic distillation. The large excess is necessary because monochlorobenzene reduces the perchloroethylene in the methanol-perchloroethylene azeotropic composition. The large excess of methanol increases the heat requirements of the distillation, thereby making this method uneconomical. Other azeotropic agents have been tested and found to demonstrate similar problems to the problem described for methanol.
It is an object of this invention to provide a novel and useful process for the purification of monochlorobenzene and in particular to provide a method for the removal of partly chlorinated aliphatic compounds from monochlorobenzene compositions. It is a further object of this invention to provide a simple and economical process for the removal of perchloroethylene and/or trichloroethylene from monochlorobenzene compositions.